1. Field of the Invention
The present invention relates to a pump light assembly for a disc laser, comprising: a focusing device, in particular a concave mirror, having a reflecting surface for focusing a pump light beam onto a laser-active medium, and a deflecting assembly for deflecting the pump light beam between a plurality of reflecting regions formed on the reflecting surface and disposed in different angular regions about the central axis of the reflecting surface. The invention also relates to a disc laser having such a pump light assembly, and to a method for pumping a laser-active medium, comprising: repeatedly focusing a pump light beam onto the laser-active medium by means of a focusing device, in particular a concave mirror wherein between successive focusing steps a deflection of the pump light beam between different reflecting regions of a reflecting surface of the focusing device which are disposed in different angles about the central axis of the reflecting surface is taking place.
Disc lasers have a laser-active medium (amplifier medium) of low thickness (laser disc) that can be cooled well. The concept of the disc laser is therefore suitable for high laser powers in the multi-kilowatt range. Owing to the low thickness of the amplifier medium, however, little pump radiation is absorbed during a pass through the laser-active medium and, without the provision of suitable measures in the pumping of the laser-active medium, this results in a low efficiency of the laser system. To obtain a minimum energy or minimum laser power required to fulfill the laser condition in the laser-active medium, a multiple pass by the pump radiation is generally required.
That multiple pass may be accomplished, for example, in the manner described in EP 0 632 551 B1. Therein, a pump light beam is radiated in at an angle to the top side of the disc-shaped laser-active medium and is reflected at a reflecting surface provided at the rear side of the laser-active medium. The reflected pump light beam is subsequently reflected back onto the laser-active medium repeatedly via a plurality of pump light mirrors and auxiliary mirrors. For the deflection, spherical mirrors are used and a direct 1:1 image of the pump spot is produced in two successive focusing steps. However, the pump beam diverges with every imaging operation, with the result that the size of the pump spot increases and hence the power injected into the laser-active medium decreases, which limits the number of the multiple passes.
2. Description of Related Art
The pump assembly described in EP 1 252 687 B1 is intended to solve that problem, see also “Pumpoptiken and Resonatoren für Scheibenlaser” (pumping optics and resonators for disc lasers), S. Erhard, Dissertation, University Stuttgart, 2002, ISBN 3-8316-0173-9. Therein, the pump light beam is focused onto the laser-active medium with the aid of a parabolic mirror, the multiple pass being achieved by means of the pump light beam being repeatedly deflected with the aid of deflecting means, for example prisms, between different reflecting regions lying in different sectors in one and the same ring region of the parabolic mirror. Since the parabolic mirror collimates the divergent pump light beam emerging from the laser-active medium which is disposed in the focal plane of the parabolic mirror, the divergence of the pump radiation field to be focused is partially compensated for during the multiple pass, with the result that a higher pump power density can be produced in the laser-active medium.
The deflection of the pump radiation between different sectors of the parabolic mirror does, however, mean that, as the number of passes increases, the utilization of the reflecting surface formed on the parabolic mirror decreases, that is to say, increasingly smaller angular regions (sectors) with increasingly smaller reflecting regions are required for reflection of the pump radiation. The consequence of this is that an increase in the efficiency of the pump light assembly, which becomes necessary at high laser powers, can be achieved only by a higher beam quality or a smaller beam parameter product of the pump light.
In “Scheibenlaser mit Kilowatt-Dauerstrichleistung” (disc lasers with kilowatt continuous-wave power), C. Stewen, Dissertation, Universitat Stuttgart, 2002, ISBN 3-89675-763, it is proposed that, using a plurality of deflecting units, also a plurality of ring regions with reflecting regions on the parabolic mirror be utilized for imaging, the individual rings being disposed radially in a segment of the parabolic mirror. The imaging of the pump light spot typically takes place in this case with telecentric imaging.
The greater the number of deflections of the pump light beam, the greater becomes the effect of divergence, which cannot be compensated for by the use of the parabolic mirror. This leads to a noticeable enlargement of the pump spot or of the collimated bundle of rays between the parabolic mirror and the deflecting devices, so that, even in the case of such a pump light assembly, the number of multiple passes and hence the level of the pump power density in the laser-active medium is limited.
The dissertation of S. Erhard also proposes using a telecentric imaging for the production of multiple passes in which the distance between the two lenses used for the imaging corresponds to the sum of their focal lengths. In real systems a (possibly undesired) curvature of the surface of the pumped laser disc and/or a thermal lens occurs which lead(s) to a resulting total focal length fges that, despite the arrangement of the lenses at the distance of the sum of the focal lengths (2 f), leads to an expansion of the beam in the imaging. In the dissertation, it is proposed that that effect be compensated for by introducing a small deviation δ of the distance of the lenses of double the focal length, which is selected in such a way (δ=f2/fges) that the overall system makes telecentric imaging possible.
However, in a laser system, owing to the focusing device, aberrations occur which add up with every pass. Those aberrations therefore limit the number of multiple passes and hence the level of the pump power density in the active medium. Those aberrations cannot be described with an additional lens and were not considered in the dissertation.